On Wed, 15 Aug 2007, Segher Boessenkool wrote:

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What you probably mean is that the compiler has to assume any code
it cannot currently see can do anything (insofar as allowed by the
relevant standards etc.)

I think this was just terminology confusion here again. Isn't "any code
that it cannot currently see" the same as "another compilation unit",

It is not; try  gcc -combine  or the upcoming link-time optimisation
stuff, for example.

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and wouldn't the "compilation unit" itself expand if we ask gcc to
compile more than one unit at once? Or is there some more specific
"definition" for "compilation unit" (in gcc lingo, possibly?)

"compilation unit" is a C standard term.  It typically boils down
to "single .c file".

As you mentioned later, "single .c file with all the other files (headers
or other .c files) that it pulls in via #include" is actually "translation
unit", both in the C standard as well as gcc docs. "Compilation unit"
doesn't seem to be nearly as standard a term, though in most places it
is indeed meant to be same as "translation unit", but with the new gcc
inter-module-analysis stuff that you referred to above, I suspect one may
reasonably want to call a "compilation unit" as all that the compiler sees
at a given instant.

BTW I did some auditing (only inside include/asm-{i386,x86_64}/ and
arch/{i386,x86_64}/) and found a couple more callsites that don't use
cpu_relax():

arch/i386/kernel/crash.c:101
arch/x86_64/kernel/crash.c:97

that are:

	while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {
		mdelay(1);
		msecs--;
	}

where mdelay() becomes __const_udelay() which happens to be in another
translation unit (arch/i386/lib/delay.c) and hence saves this callsite
from being a bug :-)

Curiously, __const_udelay() is still marked as "inline" where it is
implemented in lib/delay.c which is weird, considering it won't ever
be inlined, would it? With the kernel presently being compiled one
translation unit at a time, I don't see how the implementation would
be visible to any callsite out there to be able to inline it.


Satyam